The present invention relates to a carbonaceous char which has utility for the removal of cationic metals from aqueous and other non-organic media.
This invention relates to a carbonaceous char for the purification of aqueous and other non-organic media. The purification is accomplished by using a char of the present invention to remove metals which contaminate the media. Typical metals include mercury, lead, copper, manganese, nickel, and cobalt, which exist in such media primarily as bivalent metal ions. Even at very low concentrations many of these metals are toxic to life and require removal prior to use in potable water systems. Carbonaceous chars, such as activated carbons and charcoals, have been used extensively for the removal of toxic organic compounds from such aqueous streams, but have been found to have only limited utility for the removal of metals.
To improve the metals-removal capabilities of a char, a number of methods have been employed in the prior art. For example, organic chelants such as ethylenediamine tetraacetic acid (EDTA) and nitrilotriacetic acid (NTA) are used to bind the metal to the carbon. These chelants may be physically adsorbed onto the carbon initially or added to the aqueous solution prior to contacting the solution with the char. The disadvantages to this approach include the high costs of the chelants and their relative instability on the carbon surface.
In other methods, the carbon surface itself is treated to enhance the metals-removal properties of the carbon surface. For example, it well known in the prior art that oxidation of the carbon surface is capable of improving the metals-removal ability of a carbon material. Other modifications of the carbon surface have included treatments with sulfur compounds or the direct chemical attachment to the carbon surface of functional groups which are thought to have metals-removal properties. Depending on the type of surface modification attempted, the mechanism of metals removal may vary. In some cases, an ionic exchange mechanism is believed to characterize the process, while in other cases the mechanism appears to be one which is more covalent and chelating in nature. In many other cases, the exact mechanism of metals removal is unknown and the subject of competing theories.
In all of the prior art, few useful guidelines have been established by which the intrinsic properties of a carbon char can be used to establish the effectiveness of a given char for metals removal. In most cases, the carbon material is tested directly for the removal of the cation of interest. Although the oxygen content of the char has been cited by some as a compositional parameter important to metals removal, it has been shown that not all oxygen bound to the carbon surface is active for metals removal. Some functional groups such as ether, lactone, and carbonyl, have little relevance in and of themselves to metals removal. Similarly, titration values for surface acidity have been suggested as an indicator, but not all measured surface acidity may be useful for metals removal. In some treated carbon chars, the majority of the measured acidity may be present merely as free acid and serve no useful purpose.
Therefore, it is the object of the present invention to provide a carbonaceous char for the removal of metals from aqueous and other non-organic media which is intrinsically active and highly effective for metals removal. It is further the object of the present invention to identify and measure that property of the char which can be most effectively related to metals removal in this application.
The present invention provides for a carbonaceous char for the removal of metals from aqueous and other non-organic media. Chars effective for this purpose have a normalized contact pH less than 3 and preferably equal to or less than 2.5. The utility of the char for this application has been found to correlate with a normalized contact pH of the char. At normalized contact pH values above about 4, even a large decrease in pH often provides only a marginal improvement in the metals-removal capability of the char. However, below a normalized contact pH of around 2.5, the metals-removal capabilities of the char can improve dramatically with even a small additional decrease in pH.
For the purpose of the present invention the term xe2x80x9cnormalized contact pHxe2x80x9d means the pH of a carbon char measured by contacting the char with an aqueous solution of a salt of a strong acid and a strong base, and then measuring the pH of the fluid in contact with the carbon after a period of time sufficient to establish a relatively stable pH reading. Prior to measurement of the normalized contact pH, however, the char is prepared first by oven-drying it and then contacting the dried char with an aqueous solution of a strong, non-oxidizing mineral acid, preferably a volatile mineral acid such as hydrochloric acid, to remove interferents which may mask or compromise the intrinsic ability of the char surface to remove metals from aqueous solution. Following this procedure, the char is rinsed with deionized water to remove any free acid from the surface, and then oven-dried to remove any residual water from the char. The char resulting from this procedure is then allowed to contact, with stirring, the aqueous solution of a salt of a strong acid and a strong base as described above.
Various ratios of char weight to solution volume and concentration may be chosen to measure the normalized contact pH of the carbon char/solution mixture, but the choice will affect the final value of the measured normalized contact pH since pH is a concentration-dependent parameter. However, the parameters which give rise to the observed pH are intrinsic properties of the carbon surface which have a direct bearing upon the metals-removal capabilities of the char. It is not necessary to know the precise nature of these surface properties, i.e. which features of the carbon surface are responsible for the observed metals removal. However, it is necessary to normalize these surface features such that meaningful comparisons can be made of the metals-removal capabilities among different carbon materials. Since hydrogen ion is likely to be the most readily displaced of all cations, and since a large excess of hydrogen ion can remove other cations which may compromise the maximum intrinsic metals-removal ability of the char, prior exposure of a char to hydrogen ion can be used to provide a common, or xe2x80x9cnormalizedxe2x80x9d basis for the comparison of the metals-removal abilities of one char to another provided any free acid is removed from the char after said exposure.